So this one worked! The first one didn't, though Friendica automatically parsed the forum tag in that one and posted it on my stream as if it was posted to the forum. This one, the forum tag is not parsed (it still shows up as ! { gplus2 @ zotum . net }) and it's not displayed as if it was in a forum, and yet it's showing on Hubzilla. Weird...

@Shelenn Ayres That seems to be the current syntax to allow a non-native user to post to a forum on another service (e.g., a Friendica user posting to a Hubzilla forum, or a Diaspora user posting to either one).

Diaspora only recognizes the @ type of mention, so it leaves a bang (!) mention unparsed.

However, it does seem possible to post to a Hubzilla forum (and perhaps a Friendica forum) by starting a post with ! { forum @ pod }, assuming you have first become a member (by following forum @ pod, and the forum follows back).

My testing using hubzilla has produced...somewhat strange and hard to interpret results. A lot of it seems to work, but a post from diaspora seems to only propagate to other diaspora pods, and a lot of comments just don't propagate.

And as far as I can tell, no one has even tried posting from Friendica to a Hubzilla forum.